On-line shape inspection using a projected fringe pattern

Sammanfattning: A stereo camera system together with a single projected fringe pattern and one image recording is used to measure 3D-shape. The idea is to make the measurement quick enough to be able to use the method when testing quality on-line in manufacturing industry. For this to be possible the digital master (e.g. the CAD model) of the object is used to find homologous points, i.e. points corresponding to the same object point, in the cameras. Given the homologous points the shape can be calculated using optical triangulation. In the results of the shape measurement it can be seen that as long as the object has approximately the same shape as expected (the same shape as the digital master) the method works. For the setup used in these measurements the accuracy was ≈± 40 µm. However, large deviation from the digital master will be incorrectly detected due to the fact that the phase in the images is wrapped. An analytical expression for the maximum detectable deviation is calculated and for the setup used it became  ≈± 1.6 mm. Since the accuracy in the shape measurement depends on the phase error an analytical expression for this is derived. It is seen that the phase error becomes large if the fringe modulation is low and the speckle contrast in the fringe pattern is high. The phase in the fringes is determined using a Fourier filter method which gives that the phase error also depends on the size of the filter in the Fourier spectrum. To reduce the phase error focus has been put on suppressing the speckles by moving the aperture. It is verified by simulations and experiments that this method reduces both the speckle contrast and the phase error by 60 % for a movement of three aperture diameters.